The invention relates to a transparent shrinkable film comprising a base layer prepared from propylene-containing polymers and a hydrocarbon resin. The invention also relates to a process for the manufacture of a film of this type and its use as shrink labels.
From "Verpackungsrundschau" ("Packaging Review"), No. 10/1983, pp. 1121-1122, it is already known to surround cylindrical or generally cylindrical packaging units, such as bottles or cans, with overall labels covering their entire circumference; these overall labels are termed `sleeves` in the English-language literature. The films used for this purpose are, for example, made of polyethylene, polybutylene, polystyrene, polyethylene-polypropylene copolymers, ethylenevinyl acetate copolymers and various blends, but in particular of polyvinyl chloride. To obtain shrink properties of the films in the circumferential direction of the overall labels, which are appropriate for the intended purpose, the films are in general biaxially stretch-oriented, for example, according to the bubble, stentar or calender process, particular attention being paid to orientation in the transverse direction. In order to ensure an absolutely crease-free, close contact between the overall label and the packaging unit, the shrink values required are, for example: transverse shrink (s.sub.t) about 15% to 40% and longitudinal shrink (s.sub.t) not more than 2% to 5%, at a temperature of 90.degree. C. and a treating time of 15 minutes in a circulating air oven.
Moreover, the shrink films are provided with a printing, for example by the reverse printing process, and are subsequently glued or welded to form a tubing. Due to the controlled transverse shrink, the tubing shrinks tightly and crease-free around the packaging unit in the shrink tunnel. The label is applied automatically with the aid of brushes or optionally also by hand. In addition to the desired shrink, the overall labels must possess the following properties: high gloss, optical clarity, good slip and rigidity (=product of modulus of elasticity and thickness.sup.3) for the automatic application of the label, good printability and good weldability/bondability.
For an economical fabrication of overall labels it is also advantageous to have heat-sealable films. Compared with gluing or welding, less time is required for sealing and, in addition, it is possible to save material.
As indicated in U.S. Pat. No. 4,352,849, all requirements outlined above are, according to the state of the art, best attained by PVC film. It is in particular due to the high shrinkability and the good optical and mechanical properties of these films that almost the complete application range is covered. The variation in shape or diameter of the packaging unit, in particular of the can or bottle, can amount to more than 30% in the area of the label.
Disadvantages of overall labels made of PVC include high price, which mainly results from the high density of PVC of 1.39 kg/dm.sup.3, which is about 50% higher than the density of, for example, polypropylene. Furthermore, there are problems with regard to corrosion of the manufacturing and processing equipment (see EP-A No. 0 233 400). Polyolefin-based films for shrink labels are predominantly formed of blends comprising homo-, co- and terpolymers. To achieve the required shrink properties, the films are biaxially stretched in a bubble or stenter process.
As is known from the prior art, shrink labels based on polyolefin films have properties which may prove disadvantageous during processing for practical use. In most cases, their mechanical properties, such as tear resistance, modulus of elasticity and transverse thickness profile, are poor. The process for accomplishing high transverse shrinkage and low longitudinal shrinkage furthermore is very expensive. Moreover, the films are non-sealable.
Insufficient mechanical properties are a particular disadvantage when automatically operating shrinkage equipment is used. To obtain an adequate rigidity, comparable to that of PVC (corresponding or similarly good), the thickness of polyolefinic film must, in some cases, be increased by up to 50% or even more. As a result of this, the price of the film rises correspondingly and, moreover, the shrink rate (to attain an identical final shrink value) is slowed down by the increased film thickness.
A prior art process for achieving a high ratio of transverse shrinkage to longitudinal shrinkage in polypropylene films is, for example, described in detail in EP-A No. 0 171 733. This publication also discloses films forming the class of films comprising propylene-containing polymers and a resin addition. In the process described in this publication, a biaxially oriented film is produced in a two-stage process. To achieve a low longitudinal shrinkage the film is subjected to an additional tempering treatment at elevated temperature (about 130.degree. C.) between the longitudinal stretching and the transverse stretching. The tempering time ranges between 2 and 180 seconds. Depending on the operating speed of the production line (nowadays usually about 200 to 300 m/min) the process requires the installation of a relatively expensive and complicated aggregate between the sections for longitudinal and transverse stretching, since at a tempering time of, for example, 60 seconds and a machine speed of 200 m/min, the oven would have to be 200 m long. From the point of view of process technology, the method is therefore hardly suitable for film production according to modern processes. Moreover, the mechanical properties of the film need improving and it is, in particular, the uniformity of thickness measured over the film width which is unsatisfactory due to the very low transverse stretching ratio of about 4. Uniform winding into reels and a satisfactory printing image cannot be obtained.
It is also known that, in addition to PVC, blends based on olefinic homo-, co- and terpolymers are especially used for application as overall labels. Thus JP-A No. 224 868 (Toray) describes a heat-shrinkable multilayer film based on polypropylene, which has an AB or ABA layer arrangement. Layer A comprises a random ethylene-propylene copolymer and layer B a blend of a random ethylene-propylene copolymer and a propylene-butylene copolymer. As is specified, the shrinkage of the film in the circumferential direction of the finished sleeve is s.sub.t .gtoreq.15% at 100.degree. C. This film, however, has the disadvantage that the values of its mechanical properties are strikingly low. While in the case of PVC, the values of the modulus of elasticity are about 2,600 to 2,900 N/mm.sup.2 in the longitudinal direction (E.sub.1) and about 3,500 to 3,800 N/mm.sup.2 in the transverse direction (E.sub.t), the corresponding values of the above-described polyolefinic film are E.sub.1 about 100 to 600 N/mm.sup.2 and E.sub.t about 100 to 1,500 N/mm.sup.2. To achieve an identical or similarly good rigidity as in PVC, the thickness of the polyolefinic film must be increased by about 50%. This has, however, the consequence that the price of the film rises correspondingly.